Final answer:
External users need at least the Customer role to access a ServiceNow instance. This role allows them to perform customer-oriented tasks within the system.
Step-by-step explanation:
To access a ServiceNow instance, external users must maintain, at a minimum, the Customer role. This role provides the necessary permissions to interact with the ServiceNow system in a limited capacity commonly required for tasks relevant to customer service and support interactions.
Any roles beyond the Customer role would typically relate to specific additional permissions or access rights as determined by the organization's administrators and tailored to business needs.